: The long-term objective of the proposed work is to develop "test kits" for accurate measurement of absorption of nutrients of interest to human health based on collection and analysis of a single stool sample. In this approach the investigators will combine nutrients labeled with stable isotopes with a novel nonabsorbable marker.Initially they will focus on development of the approach for fat absorption in children with cystic fibrosis - a population for whom such a test would be an important clinical management tool. During Phase II they will conduct in-depth investigations into the details of the procedure for use with two key groups of nutrients: fat and minerals. The goal of this Phase is the development of "test kits" for accurate measurement of absorption of fat and/or selected minerals without the need for quantitative stool collection. Most of this work will be conducted in cystic fibrosis patients, however, directed studies will also be performed in other patient populations for whom malabsorption is a key parameter. During Phase III, the procedures and "test kits" developed will be made available to the community at-large. A central laboratory will be organized at BioChemAnalysis to provide analytical and other support facilities to serve the community.